MADRID (AP) — More than two dozen passengers from at least 12 different countries left a cruise ship hit by a deadly hantavirus outbreak without contact tracing nearly two weeks after the first passenger died on board, the ship operator and Dutch officials said Thursday.

WATCH: News Wrap: 3 new patients evacuated from cruise ship with deadly hantavirus outbreak

Health authorities on at least four continents are now tracking down and in some cases monitoring the cruise passengers who returned home on April 24, and trying to trace others who may have come into contact with them.

Experts say the risk to the wider public is considered low because hantavirus — usually spread by the inhalation of contaminated rodent droppings — isn't easily transmitted between people.

WATCH: Can hantavirus spread between humans? What to know as WHO investigates ship outbreak

The Dutch health ministry said Thursday that a flight attendant on a plane briefly boarded by an infected cruise passenger in South Africa was showing symptoms of hantavirus and would be tested in an isolation ward at a hospital in Amsterdam. The cruise passenger, also a Dutch woman, was too ill to fly and was taken off the plane in Johannesburg, where she died.

If the woman tests positive, she could be the first known person not on the MV Hondius to become infected in the outbreak.

Three cruise ship passengers have died in the outbreak, and several others are sick. Symptoms usually show between one and eight weeks after exposure.

1st hantavirus case on board was confirmed May 2

Three people, including the ship's doctor, were evacuated Wednesday while the ship was near the West African island country of Cape Verde and taken to specialized hospitals in Europe for treatment.

READ MORE: Cruise ship with hantavirus outbreak heads to Canary Islands after 3 people evacuated

The body of the Dutch man who was the first to die on board on April 11 was taken off the ship on the remote South Atlantic island of St. Helena on April 24, when his wife also disembarked. She then flew to South Africa a day later and died there.

Oceanwide Expeditions, the Netherlands-based cruise ship company, said Thursday that 29 passengers left the vessel at St. Helena, while the Dutch Foreign Ministry put the number at about 40. The company had not previously said that dozens more people left the ship on April 24.

It wasn't until May 2 that health authorities first confirmed hantavirus in a passenger on the ship, the World Health Organization has previously said. That was in a British man evacuated from the ship to South Africa from Ascension Island three days after the St. Helena stop. He was tested in South Africa and is in intensive care there.

The people who left the ship at St. Helena to return to their home countries were of at least 12 different nationalities, Oceanwide Expeditions said. It said there were also two people whose nationalities were unknown.

It emerged Wednesday that a man tested positive for hantavirus in Switzerland after he also disembarked at St. Helena and flew home, though his precise movements aren't clear.

On Thursday, Singaporean health authorities said they were monitoring two men who got off the ship at St. Helena and flew to South Africa and then home. The two men, who arrived in Singapore at different times, were being tested for hantavirus and were isolated at the country's National Center for Infectious Diseases, officials said.

One had a runny nose and the other had no symptoms, Singapore's Communicable Diseases Agency said.

British health officials say two people who were passengers aboard the ship but flew home midway through the journey are self-isolating but do not have symptoms of illness. The U.K. Health Security Agency said "a small number" of contacts of the two are also self-isolating but are not showing any symptoms. Other contacts are being traced.

Authorities in St. Helena, the remote, volcanic British territory in the South Atlantic where passengers got off, said they were monitoring a small number of people who were considered "higher risk contacts." Those higher risk contacts were being told to isolate for 45 days, the St. Helena government said.

South Africa is tracing contacts from an April 25 flight

Meanwhile, the vessel is now sailing to Spain's Canary Islands, a voyage that is expected take three or four days, with more than 140 passengers and crew members still on board.

WHO Director-General Tedros Adhanom Ghebreyesus said Thursday that he had "been in touch with the ship's captain regularly, including this morning."

"He told me morale has improved significantly since the ship started moving again," Tedros said.

Authorities in South Africa are also trying to trace contacts of any passengers who previously got off the ship. They have focused mainly on an April 25 flight from St. Helena to Johannesburg, the day after passengers disembarked there.

The Dutch woman from the cruise ship who later died in South Africa briefly boarded that flight, officials have said. It's not known how many other cruise passengers also were among the 88 people on it, but flights from St. Helena go to South Africa and are rare, normally once a week.

The body of the third fatality, a German woman, is also still on board the ship after she died on May 2.

Tests have confirmed that at least five people who were on the ship were infected with a hantavirus found in South America, called the Andes virus. It can cause a severe and often fatal lung disease called hantavirus pulmonary syndrome.

Argentina's health ministry said there were 28 deaths from hantavirus last year, up from an average mortality rate of 15 in the five years before that. Nearly a third of cases last year were fatal, it said.

The Andes strain is the only hantavirus known to spread from human to human.

The ship departed from Argentina and investigations into the source of the outbreak are focusing on that country. The Dutch couple, the first passengers to fall sick, traveled there and elsewhere in South America before boarding the ship, according to WHO.

Quell reported from The Hague, Netherlands. Imray reported from Cape Town, South Africa. AP writers Jill Lawless in London and Geir Moulson in Berlin contributed.